1. Field of the Invention
The present invention relates to a thin film transistor and a fabrication method thereof.
2. Description of the Related Art
Amorphous silicon thin film transistors (hereinafter referred to as a-Si TFT) are well known as pixel switching transistors devices for use with active matrix type liquid crystal display (LCD) devices and as transistor devices which construct LCD device driver circuits integrally formed on array substrates.
In particular, active matrix type LCD devices having a-Si TFTs are preferably used for display devices such as projector type LCD devices as well as LCD type TV sets and OA devices.
FIG. 15 shows the construction of an example of a related art bottom-gate staggered a-Si TFT.
Referring to FIG. 15, the a-Si TFT comprises a transparent insulating substrate 1501, a gate electrode 1503, a gate insulating layer 1505, a semiconductor layer 1513, a source region ohmic contact layer 1515, a drain region ohmic contact layer 1517, a source electrode 1519, a source electrode 1521, and a channel-stopper insulator layer 1525. The gate electrode 1503 is formed over the transparent insulating substrate 1501. The gate insulating layer 1505 is formed over the gate electrode 1503 in such a way that the gate electrode 1503 is overlaid with the gate insulating layer de1505. The semiconductor layer 1513 is formed over the gate insulating layer 1505. The semiconductor layer 1513 having a source region 1507, a drain region 1509, and a channel region 1511 which is disposed therebetween. The source region ohmic contact layer 1515 is formed over the source region 1507. The drain region ohmic contact layer 1517 is formed over the drain region 1509. The source electrode 1519 is formed over the source region ohmic contact layer 1515 so that they are ohmic contacted with each other. The source electrode 1521 is formed over the drain region ohmic contact layer 1517 so that they are ohmic contacted with each other. The channel-stopper insulator layer 1525 is formed over a channel region 1523 of the semiconductor layer 1513.
When an a-Si TFT is used for a projector type LCD device where strong light is entered by a light source, a light insulating film which is named a black-matrix is disposed on the rear surface of the transparent insulating substrate 1501. Or disposed on the front surface of the transparent insulating substrate opposed to the transparent insulating substrate 1501.
This light insulating film prevents light from entering the channel region 1523 of the a-Si TFT so as to suppress occurrence of an optical leak current.
However, as the size of pixels of the projector type LCD device decreases, reflected light in liquid crystal cells causes an optical leak current to take place in the thin film transistor. The optical leak current results in malfunction of the LCD device.
Particularly, in the above-described bottom-gate staggered a-Si TFT, since the source electrode, the drain electrode, and the ohmic contact layer are not largely spaced apart from the channel region, reflected light which takes place at such electrodes and layer much affect the channel region, thereby causing an optical leak current to take place in the TFT. Thus, the TFT will malfunction.
On the other hand, it is known that such an a-Si TFT is preferably fabricated by self-alignment method. Among various constructions and fabrication methods of a-Si TFTs, a self-alignment type a-Si TFT has the following advantages over the other types of a-Si TFTs.
(1) Occurrence of parasitic capacitance due to overlapping of the gate electrode and source region/drain region can be prevented.
(2) On the fabrication stage, the gate electrode and the source region/drain region are self aligned. Thus, the a-Si TFT can be easily fabricated without necessity of precise alignment control.
(3) From (1) and (2), when the a-Si TFT is used for a LCD device, since the electrostatic capacitance for each gate electrode decreases, it can operate with high speed signals. In addition, since the alignment less deviates, parasitic capacitance (Cgs) between the gate electrode and the source electrode less deviates. Thus, when an LCD device with a large display area is formed, a feed-through voltage caused by gate pulses in pixel electrodes can be restricted. The countermeasures against display failure due to the feed-through voltage can be simplified.
The a-Si TFT shown in FIG. 15 is an example of the above-mentioned self alignment type a-Si TFT. In the a-Si TFT shown in FIG. 15, an n type a-Si layer and an electrode layer are formed over a positive-type photoresist (which has been patterned and deposited over the gate electrode by back-side exposure technique) by CVD method or the like. A pattern is formed by lift-off method or the like. FIG. 16 shows the construction of another example of a-Si TFT. This a-Si TFT is fabricated in the following manner. A metal film (such as Al film) for a source electrode and a drain electrode is reacted with an a-Si film over a semiconductor layer 1513 made of n type a-Si doped by ion implantation method. Thus, a silicide film 1601 is formed.
The specific resistance of the n type a-Si layer of a-Si TFT on which the source region and drain region are formed is approximately 10.sup.2 .OMEGA.-cm. The sheet resistance of the n type a-Si layer with a thickness of 500 angstroms is 2.times.10.sup.7 .OMEGA./.quadrature.. Thus, the electric resistance of the n type a-Si layer is very high. To lower the resistance, in the a-Si TFT, the n type a-Si layer is often used as an ohmic contact layer. An electrode layer made of a conductor (such as a metal) is deposited over the n type a-Si layer so as to lower the resistance.
However, in the a-Si TFT having an ohmic contact layer which is a deposited n type a-Si layer or the like, when a photoresist used for etching out a channel-stopper insulator layer is patterned by back-side exposure technique, the transmittance of light of the ohmic contact layer is insufficient. Thus, the exposure of the photoresist is insufficient. In addition, the exposure process takes a long time and developing conditions become strict. Consequently, the yield and throughput on fabrication stage are deteriorated.
When the ohmic contact layer and the electrode layer are patterned by lift-off method, incomplete lift-off takes place, thereby deteriorating the yield.
To solve such a problem, an electrode forming method using a silicide has been proposed.
However, when a silicide is thinly formed, the resistance becomes unstable. In contrast, when a silicide is thickly formed, it erodes the channel region of the thin a-Si, thereby deteriorating the characteristics of the TFT. In addition, when the electrode layer is formed of a silicide, an off-current may sometimes increase.
Thus, an electrode layer is not easily formed of a silicide.
The present invention has been made to solve such problems. An object of the present invention to provide a thin film transistor (TFT) which is used in a projector type LCD device and the like, which does not deteriorate the yield and throughput on the fabrication stage, and which restricts an photo leak current caused by reflected light at a source electrode, a drain electrode, and an ohmic contact layer.